The present invention relates to a sealing arrangement.
When producing paper pulp from cellulose-containing fibre material, it is necessary to wash and dewater the paper pulp at a number of stages in the process.
A previously known and commonly used arrangement for washing and dewatering paper pulp, called a wash press, is disclosed in SE-C-380,300, SE-C-501,710, U.S. Pat. No. 5,488,900 and SE-C-504,011. The arrangements disclosed in these documents comprise two cylindrical, rotatable screen members arranged in an essentially converging vat. Other examples of known arrangements are disclosed in U.S. Pat. No. 4,543,161 and in U.S. Pat. No. 5,667,642, the latter constituting an arrangement in which the screen members rotate in the opposite direction to the conventional one, that is to say, as seen from the short end, the right-hand screen member rotates counter-clockwise and the left one rotates clockwise.
A problem encountered in washing and dewatering with wash presses of the above mentioned type is that a very high local pressure is built up at the end of the screen drum, which acts on the sealing arrangement at the end wall. U.S. Pat. No. 4,543,161 discloses two alternatives for handling the high pressures in the nip. In a first variant, shown in FIGS. 3-4, the ends of the drums are given a slightly smaller dimension, for which reason the pulp dewatered in the nip can be pressed out into a collection shaft which is formed in the area of reduced diameter of the drum. In this way, the high pressure at the end wall of the screen drum is reduced.
A second variant, shown in FIGS. 5-7, is a variant with low-friction shields which bear against the end walls of the drum in the area of the nip and prevent the pulp from being pressed out to the sides in the nip. This type of seal becomes greatly worn because the very high local hydraulic and mechanical pressures against the ends of the drum in the smallest cross section of the nip are maintained. These solutions reveal two extremes of the sealing problems, one solution resulting in a lower dry substance content of the wash press, and the other solution still resulting in very high pressures at the end wall of the screen drum.
An object of the present invention is to permit an improved sealing of the end walls of the wash press by means of controlled pressure relief at the end walls of the screen drum.
A further object is to make available a wash press with improved sealing of the end walls, which sealing allows fibre residues, which may be pressed into the pressure relief zone, to be continuously returned to the level of the envelope surface downstream of the nip. In this way, it is possible to avoid successive build-up of an increasingly hard dewatered fibre plug. The wash press can in this way be operated for longer periods of time without unnecessary shutdown for cleaning, or without the need for complicated cleaning arrangements.
Yet another object is to make available a wash press with improved sealing of the end walls, which sealing consists of a sealing profile, preferably with a pressurized seal, which affords a cost-effective sealing construction.
Yet another object of a preferred embodiment with pressurized sealing is that the pressure can be kept at a constant level in the whole sealing strip, while at the same time achieving an optimum sealing capacity around the entire circumference of the end wall. The pressure relief in the area of the nip means that one and the same pressure can be applied in the seal and still be sufficient in the area of the nip where the press forces on the pulp web are extremely high. At the same time, a contiguous sealing element can be used with just one pressurizing chamber. The seal does not therefore need to be divided into parts, pressurized at different levels, and this reduces the costs of the seal and of the pressurizing system for the sealing strip.